Question: 9 rubber stamps cost $13.68. Which equation would help determine the cost of 11 rubber stamps?
Solution: There are several equations that could help determine the cost, each with a slightly different approach. We know the cost of 9 rubber stamps. We want to know the cost of 11 rubber stamps. We can write the numbers of rubber stamps as a proportion: $\dfrac{9}{11}$ We know 9 rubber stamps costs $13.68. We can let $x$ represent the unknown cost of 11 rubber stamps. The proportion of these costs can be expressed as: $\dfrac{\$13.68}{x}$ The cost changes along with the number of rubber stamps purchased, and so the two proportions are equivalent. $\dfrac{9}{11} = \dfrac{\$13.68}{x}$